A number of cooling applications require that a fan continue to run even after the heat generating element being cooled is turned off. For example, high intensity projector light systems typically are cooled by fans that run for a pre-set time period after its light source (e.g., a high wattage light bulb) is turned off. Without this extra cooling, the excess heat generated by the light source could immediately damage itself and/or other components of the overall system. Moreover, this excess heat also can shorten the overall lifetime of the system.
Accordingly, to avoid these problems, fans used for these applications commonly have a delay circuit that continues to power the fan for a (limited) pre-set time period after the light source is turned off. In other words, even though the light source no longer receives power, the fan continues to receive power (i.e., from the power source that powers the overall system) for the noted pre-set time period. After this time period has elapsed, the fan also turns off.
Problems arise, however, during a power loss. Specifically, if there is a power loss, then both the fan and light source turn off at the same time. In other words, even though the fan includes the noted delay circuit, it still is not energized and thus, cannot rotate the rotor to generate cooling air flow. Consequently, the excess heat generated by the light source can damage the overall system.